(1) Field of the Invention
The present invention relates to non-destructive testing and particularly to the non-invasive examination of soft tissue and body organs. More specifically, this invention is directed to medical ultrasonic equipment and particularly to pulse-echo body scanners employing driven mirrors. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
Apparatus and techniques which permit the non-invasive examination of bodies through the use of ultrasonic energy are well known in the art. A persistent problem in prior art ultrasonic test equipment is that spurious signals may provide false, inaccurate or unclear readings.
One source of spurious energy in an ultrasonic transceiver of the type employed for medical diagnostic purposes is beam "displacement" which may occur when the focused ultrasonic energy, which is transmitted through a liquid, is incident upon the liquid-solid interface at the mirror employed to scan the beam over a target area of interest. The principle of "displacement" of an ultrasonic beam when it is reflected from a liquid-solid interface is disclosed in Breazeal, Adler and Flax, "Reflection of a Gaussian ultrasonic beam from a liquid-solid interface", J. Acous, Soc. Am., Vol. 56, No. 3, September 1974. As set forth in this article, "beam displacement" occurs when the angle of incidence of the beam is the same as or near the angle of excitation of surface waves on the interface. Experimental test results reported in the aforementioned article show that the energy of the reflected beam may be redistributed into two or more separate beams which are displaced from one another. As will be appreciated by those skilled in the art, redistribution of the energy incident on the scanning mirror into two or more displaced beams is undesirable. The thus "displaced beams" will be reflected from the body to be tested and, after redirection by the mirror, will be incident on the transducer crystal. Since the original and "displaced" energy will travel different distances, the signal transduced by the receiver crystal is likely to lack clarity. Thus, it is the principal object of the present invention to reduce or avoid reception of spurious ultrasonic energy by the receive transducer of an ultrasonic test device.